LTC1688 Linear Technology, LTC1688 Datasheet - Page 9

• Manufacturer Part Number: LTC1688
• Description: 100Mbps RS485 Hot Swapable Quad Drivers
• Manufacturer: Linear Technology

APPLICATIONS
High Speed Twisted-Pair Transmission
Data rates up to 100Mbps can be transmitted over short
distances using Category 5 UTP (unshielded twisted pair).
The cable distance will determine the maximum data rate.
Figures 11 and 12 show an 8ns pulse propagating over 25
feet of Category 5 UTP. Notice how the cable attenuates the
signal. Lucent Technologies’ BRF2A and BRS2A receivers
are recommended for these ultrahigh speed applications.
Figure 11. 8ns Pulse Over 25 Feet Category 5 UTP
5V/DIV
2V/DIV
2V/DIV
2V/DIV
RECEIVER INPUT
RECEIVER OUTPUT
U
DRIVER INPUT
DRIVER OUTPUT
INFORMATION
U
10ns/DIV
1/4 LTC1688
W
DRIVER
Figure 13. 100Mbps Backplane Transmission
1688/89 F11
U
TRANSMISSION LINE
BACKPLANE
High Speed Backplane Transmission
The LTC1688/LTC1689 can be used in backplane point-to-
point and multipoint applications. At high data rates,
signals should be routed differentially and PC traces
should be terminated (see Figure 13). Note that the RS485
specification calls for characteristic impedances near 100 ;
therefore, PC trace transmission lines should be designed
with an impedance close to 100 . If trace impedance is
much less than 100 , and the trace is double terminated,
the part will experience excess heating. The propagation
delay could then fall outside the specified window. The
LT1720 dual UltraFast
high data rate backplane applications.
UltraFast is a trademark of Linear Technology Corporation.
1/4 LTC1688
100
DRIVER
Figure 12. 100Mbps Differential Data Connection
RECEIVER
1/2 LT1720
100
25 FT CATEGORY 5 UTP
1688/89 F13
TM
LTC1688/LTC1689
comparator is a good choice for
100
+
–
RECEIVER
1688/89 F12
9